Started my kid on X-mart bikes, and he broke them all. Got him a Specialized Rock Hard, which weighed WAY more than he did, and he never managed to break it. Our neighbor's kid rode it down the switch-back hill once and hit a tree full bore. She got 3 stitches in her eye-brow, you can still seee the scar in the tree, but the bike? A little truing of the front tire.
IPLAYOUTSIDE.com had a picture of him with his 50 lb bike during a wintertime mtb cyclocross with the caption "what if your bike weighed more than you?" He still won the race.
As for wieght, it just makes the child a stronger rider. A guy at the LBS advised my NOT to put this kid on a bike with suspension, as his muscles would develop better without them (gosh, I would have bought him a new bike!) My kid has won 1st place at NORBA in the XC at Snowshoe in his class from 2000-2005, been on real bike teams, and is really strong on the uphill.
So, I think the wieght issue boils down to what the parent wants to lug around - the little kids don't care.